Temperature measurements of air are important in many situations, particularly when temperature data is being gathered for weather information. The historic method for acquiring air temperature data is to read a mercury-type thermometer, then record that datum value. The more modern method for acquiring such temperatures is to use an electronic sensor and then have the corresponding value logged into some type of recording device or electronic memory.
Once an electronic temperature sensor is utilized, the datum value can be automatically recorded, either locally, or via a communication link to a remote recording device. The major problem in the use of electronic temperature sensors is that, in many cases, they have proven to be inaccurate when measuring air temperature. Once example of such an inaccurate device is a $5000 electronic thermometer used by the National Weather Service in Tuscon, Ariz., which gives readings that are two degrees too warm during the day and one-half degree too cool during the night.
Another problem in the use of electronic temperature sensors is the design of the construction of many of the present temperature sensing installations. A popular device at the present time is a thermistor-based Maximum-Minimum Temperature System (MMTS), which is mounted in a small "beehive-like" structure. The MMTS is presently used by about half of the cooperative stations that observe temperature in the United States. Unfortunately, the MMTS stations give different readings than the previously used liquid-in-glass maximum and minimum thermometers mounted in Cotton Region Shelters (CRS). Such differences in readings are probably due to the different microenvironments within the CRS shelters.
An additional problem in the use of MMTS stations is that their durability and reliability have been low. Over the past six years there have been as many reported MMTS malfunctions as there are units installed. Some reasons given for such malfunctions are lightning strikes, animals cutting buried cables, and insects nesting in the shelter (which would ruin the airflow through the beehive-like shelter, which is necessary for its proper function).
An unusual temperature sensing device was disclosed in the book Instruments for Physical Environmental Measurements, by J. Y. Wang and Catherine M. M. Felton (1983). This device, referred to as a Reflecting Disc Shield, consists of two parallel reflecting disks with a temperature sensor located between the disks. The surfaces of the disks that face the temperature sensor are painted black, and the surfaces that face away from the sensor are painted white. The disks act as heat sinks and tend to stabilize the air temperature measurements, without artificial ventilation.
The Reflecting Disc Shield, as disclosed by Wang and Felton, has a theoretical accuracy of .+-.0.5.degree. F. Data taken from actual units built according to this disclosure exhibit somewhat different characteristics, however, having an inaccuracy of 5.0 degrees F. at minimum temperature during the night, and an inaccuracy of 2.0.degree. F. at maximum temperature during the day.